As an automobile fuel tank, there is provided a resin tank in place of a metal tank in response to a request for reduction in weight, and the resin tank can be obtained by hollow blow molding of thermoplastic resin such as polyethylene, etc. In general, a resin material is easily permeated by a volatile component such as gasoline and the like as compared with metal material, a barrier layer with a less permeating property composed of EVOH, nylon or the like is ordinarily disposed to an inner layer and an intermediate layer, and there is proposed a resin tank obtained by multilayer molding with polyolefin and barrier material layers (patent documents 1 and 2).
In general, a fuel feed port for feeding fuel from outside, a return path member for returning excessive fuel from an engine and the like are disposed outside of an automobile fuel tank. Further, a wave barrier and the like are disposed inside of the tank to prevent generation of noise caused by fluctuation of liquid fuel level.
When an automobile fuel tank and the like in which parts must be assembled to the inside of the hollow body as described above, conventionally, after an outer shape of the tank is given by blow molding, a hole is formed in the hollow molded body and predetermined parts are fitted to the hole. Accordingly, in the conventional manufacturing method, a step for forming the hole is necessary, from which there has been a problem of increasing man-hours.
Further, since resin material is permeated easily by a volatile component as compared with metal, although a barrier layer with low permeating property such as EVOH (ethylene/vinyl alcohol copolymer), polyamide, or the like is provided when resin material is applied to a fuel tank. However, since parts formed of different materials have no barrier layer between them, there is a problem that a volatile gas such as gasoline and the like passes through the parts.
It is also possible to provide a part attached later with a barrier layer to prevent the gas permeating property. Even in this case, however, since no barrier layer exists in the portions of the parts welded to the hollow molded body, a gas barrier property cannot be sufficiently provided, from which a problem of gas permeation arises.
Further, to install a wave barrier inside a hollow molded body such as a fuel tank and the like, it is also possible to dent a tank wall itself inward and to use the concave wall portion as a wave barrier. In this case, however, a wall thickness of the concave portion gets thinner, then an entire wall thickness must be increased, from which a problem of an increase of weight arises.
Further, the patent document 2 discloses a resin fuel vessel formed by fusion bonding injection molded half-divided bodies contacting with each other. However, since a problem to be solved by the invention of the patent document 2 is an uneven wall thickness of a hollow body made by hollow blow molding, the patent document 2 relates to a method of forming injection molded bodies having a uniform wall thickness and forming a hollow body by coupling them. Accordingly, a technical idea of obtaining an integrated hollow body by joining half-divided bodies after assembling parts in them and decorating and processing the inner surfaces of them. Further, since the half-divided bodies molded separately are integrated later, an innermost layer must be composed of a barrier layer to prevent discontinuous barrier layer on a joint surface. When a dropping impact is applied to a hollow molded body, the inner most layer, which is most subjected to be stressed, is easy to be broken. However, since EVOH and polyamide are ordinarily inferior to polyolefin in dropping impact resistance, an arrangement having the innermost layer composed of the barrier layer is disadvantageous. Further, many steps such as injection molding half-divided bodies, releasing them from dies, heating and melting fusion bonding portions of the half-divided bodies facing to each other, and jointing them by making them contact to each other. Since fusion bonded portions of the injection molded half-divided bodies must be heated and melted again, energy efficiency is also bad. Consequently, there is need for a method of manufacturing a hollow molded body without a volatile component permeation through the body using a conventional hollow blow molding apparatus.
The patent document 3 discloses a method of cutting off an extruded parison. However, since a thick parison must be cut off before it is blow molded by the method of the patent document 3, there may be a case that it is difficult to cut off a multilayered passion. Further, since the parison is blown after it is welded again, a welded portion is not blow extended, and thus wall thickness of the hollow molded body is made uneven.    Patent Document 1: Japanese Patent Application Laid-Open Publication No. S55-163134 (1980-163134)    Patent Document 2: Japanese Patent Application Laid-Open Publication No. H10-157738 (1998-157738)    Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2002-103427